Use a unique notification per screen recording

Instead of static notification IDs for recording / processing / saved, use a
new notification for each screen recording. Also adds a group key to the
saved notifications so that they are grouped together separately from
the FGS recording notification.

Fixes: 268404371
Fixes: 278954828
Test: atest RecordingServiceTest
Test: manual
Change-Id: I8e45fc57bc7b20a0bed3360390e9e8c6e98ab887